Terraforming Mars is about making Mars earthlike enough to live freely on it. There are 4 parameters on
the game board connected to this process: Temperature, oxygen level, and ocean coverage.
Whenever you raise one of them, your Terraform Rating (TR) also increases by that much, giving you a higher income and score.
When a global parameter has reached its goal, it can’t be raised any further, and so does not increase your TR. You may still play cards and actions that increase the parameter - just ignore that part of the effect. When all three global parameters have reached their goal, the game ends after that generation (after the Production phase).
Don't forget! Increasing a global parameter also increases your TR!